Ensure that ExecPrepareExprList's result is all in one memory context.

Noted by Amit Langote.

Discussion: https://postgr.es/m/aad31672-4983-d95d-d24e-6b42fee9b985@lab.ntt.co.jp

src/backend/executor/execExpr.c

@@ -511,8 +511,12 @@ List *
 ExecPrepareExprList(List *nodes, EState *estate)
 {
 	List	   *result = NIL;
+	MemoryContext oldcontext;
 	ListCell   *lc;
 
+	/* Ensure that the list cell nodes are in the right context too */
+	oldcontext = MemoryContextSwitchTo(estate->es_query_cxt);
+
 	foreach(lc, nodes)
 	{
 		Expr	   *e = (Expr *) lfirst(lc);
@@ -520,6 +524,8 @@ ExecPrepareExprList(List *nodes, EState *estate)
 		result = lappend(result, ExecPrepareExpr(e, estate));
 	}
 
+	MemoryContextSwitchTo(oldcontext);
+
 	return result;
 }